Central pit and dome craters: Exposing the interiors of Ganymede and Callisto

Abstract

Central pit craters on the large icy satellites Ganymede and Callisto are an unusual crater class perhaps related to the unusual properties of water ice. The detailed morphology and dimensions of pit crater features differ from those of smaller, normal, complex craters on these two satellites. Pit crater depths appear to be constant regardless of diameter. Rim collapse is also restricted relative to that in lunar craters or smaller Ganymede craters. The floors of pits in craters larger than ∼60 km are occupied by smooth bright domes. Pit‐to‐crater and dome‐to‐crater diameter ratios increase linearly with crater diameter, and are indistinguishable on Ganymede and Callisto and with terrain type or surface longitude. The floors and central structures of young, bright‐rayed pit craters such as Osiris are covered by a thin uniformly bright impact melt or frost deposit. Thus, domes and pits form rapidly, on the time scale of the impact itself, rather than by long‐term, postimpact intrusion or extrusion. The bright domes in pit craters are most simply explained as the uplift and exposure of relatively ice‐rich material from depths of ∼3.5 to 5 km during impact. This process is almost directly analogous to the uplift and exposure of anamolous deep‐seated material observed in terrestrial and lunar craters. The unusual pit morphology on icy satellites may be the result of impact into crust that is mechanically much weaker at shallow depth than on rocky bodies such as the Moon. Because crater morphology is strongly dependent on ice‐rock composition, the similarity of pit and dome dimensions on Ganymede and Callisto indicates that the structure and rheology of the crusts of these bodies (down to depths of ∼10 km) are very similar and have been for several b.y. Pit crater morphology indicates that the crusts of both satellites are probably ice‐rich and differentiated.